People inflicted with isolated quadriceps (thigh muscle) weakness or partial or total paralysis of the quadriceps (knee extensors) often lack the muscle strength to walk safely without collapsing under their own weight and falling. Often these people are prescribed a knee-ankle-foot orthosis (KAFO) to prevent their knee from flexing during the stance phase of gait, the period of the walking cycle when their leg is weight bearing.
Conventional KAFOs lock the knee joint in constant full extension during walking. Unfortunately, abnormal gait patterns must be adopted by KAFO users to overcome the inability to flex the knee when the leg swings forward during the swing phase of gait. These abnormal gait patterns can lead to chronic injuries, excessive energy expenditure and cosmetic implications. Walking with a fully locked knee also limits the user's mobility and prevents them from safely and efficiently walking on inclines, stairs and uneven surfaces.
A new type of KAFO known as a Stance Control Knee-Ankle-Foot-Orthosis (SCKAFO) has recently emerged, which prevents knee flexion to provide limb support in stance, and allows free knee motion in swing. A substantial portion of the population using fixed leg braces have sufficient muscle strength in their legs to benefit from a SCKAFO, including patients afflicted with multiple sclerosis, muscular dystrophy, polio/post-polio, incomplete spinal injury, unilateral leg paralysis/paresis, trauma, congenital defects and isolated quadriceps weakness/absence.
While commercial SCKAFOs do promote a more natural gait to some extent, they suffer from functional and structural limitations. Current SCKAFOs either require the knee to be fully extended to engage the knee-joint lock ([4, 19 20] and therefore would not support the limb in stumbling with a partially flexed leg), require specific unnatural ankle angles to engage the knee-joint lock [19, 2] do not allow knee extension when locked to resist flexion during stance (preventing stair/ramp climbing), or are too heavy and bulky for many potential clients and are thus energy exhaustive, obstructive, intimidating or unattractive. Brace-users wearing these orthoses are not permitted natural leg motion and are, therefore, limited in where they can walk; are not given sufficient support in case of stumbling; or have excessive energy expenditure and thus early fatigue during ambulation due to the heavy and cumbersome design. The bulkiness of these orthoses also tends to discourage their use for cosmetic reasons.
Numerous attempts have been made over the past century to design a practical SCKAFO. Harrison et al. developed a prototype SCKAFO knee joint based on a roller clutch design [1]. As is common to most roller clutches, the rollers are contained in a cage to promote simultaneous wedging of all the rollers. The knee-joint design connected a control arm to the roller cage. Actuation of the control arm would position the cage to hold the rollers at the wide end of their respective wedge-shaped chambers. The rollers would therefore be prevented from contacting both the inner and outer race of the clutch and locking up. Pivoting the cage back to its original position, via the control arm, would return the rollers to their intended duty of providing unidirectional rotation of the concentric races. The roller clutch joint could therefore provide uninhibited movement during swing and unidirectional motion during stance. Unfortunately, the joint had an excessively thick profile and the rollers were prone to jamming into the wedges and required an impractically high disengagement force that tended to deform the cage.
As a second effort, Harrison et al. developed a wedge-joint model [1]. The joint model used a solenoid-actuated wedge, which lodged itself into the rear joint space of a polycentric knee joint during stance, thus allowing joint extension while preventing flexion. During the swing phase, the solenoid would retract the wedge from the joint space and allow free extension and flexion. It was found that an excessive amount of force was needed to retract the wedge from the joint space. The wedge also experienced significant plastic deformation due to the high, localized loads endured while preventing flexion.
As a third design, Harrison et al. developed and tested a lever-lock knee joint design [1]. The lever-lock design consisted of a ring attached to the lower portion of the orthosis that rotated freely through a hole made in an actuation bar, connected to the upper portion of the orthosis. While the hole remained perpendicular to the ring's tangent, the joint allowed rotation in both directions. When the solenoid pivoted the actuation bar, the hole in the bar would sit at an angle to the ring and the ring would jam to prevent flexion. However, the joint was considered expensive to manufacture, the joint sliding action was found to be too rough, and the ring was subject to scoring under medium loads.
U.S. Pat. No. 4,632,096 to Harris discloses a dynamic knee orthosis that unlocked following a pre-selected dorsiflexion of the ankle, followed by a pre-selected plantar flexion of the ankle [2]. The automatic locking knee joint incorporated a complex linkage system of levers and springs, using the concept of impingement to lock the knee. The design is impractical, as it requires the patient to make specific ankle movements during the gait cycle to engage the knee lock. A locking knee brace, which relies on ankle motions to engage and disengage the locking mechanism, cannot be used by people with fused, deformed or spastic ankles, and would not be suitable for stumbling. Dynamic knee joints, which rely on the concept of impingement, are often prone to jamming, and require large disengagement forces to unlock the knee if any external knee moment is present [3].
A hydraulic-based, automatic locking knee device was designed by the University of Toledo [4]. The joint system consisted of a hydraulic fluid filled bulb positioned below the heel, attached to a hydraulic line running to the knee that attached to a piston, which engaged and disengaged the knee locking mechanism. At heel strike, the bulb was compressed and the displaced hydraulic fluid would extend the piston to engage the lock. At heel off, the bulb relaxed and the piston would fall downward to disengage the lock. The locking mechanism required the knee to be fully extended in order to lock. This requirement would not provide security when climbing stairs or walking on uneven ground.
U.S. Pat. No. 5,267,950 to Weddendorf discloses a friction-based automatic-locking orthotic knee device [5]. At the knee joint, two bevelled, serrated brake plates were fixed to the lower portion of the orthosis. They were positioned just below either side of a bevelled shoe, fixed to the upper portion of the orthosis. The lower portion of the orthosis could move up and down relative to the upper portion at the knee joint. A spring at the knee joint kept the serrated brake plates and the bevelled shoe separated. The application of weight to the knee joint at heel strike thrust the lower portion of the orthosis toward the upper portion, ramming the serrated brake plates into the bevelled shoe thus jamming the shoe between the sandwiching plates. Releasing body weight at heel off allowed the joint spring to separate the brake plates from the sandwiched shoe. Disadvantages of this design include the knee joint's inability to allow extension during stance, potential jamming issues and wear of the bevelled shoe and brake plates from repeated use.
Recently a dynamic knee joint was developed that incorporated a band brake to inhibit knee flexion, as disclosed by Tokuhara et al. [6]. The device named Intelligent Brace II (IB-II) had a heel contact switch which activated the band brake, located at the knee joint. An onboard microcomputer detected resistance in the knee and calculated the optimal braking force to be imparted to the knee via a stepping motor. Weighing well over 3.7 kg (8.2 lbs), the IB-II is too heavy for practical application. A practical SCKAFO must weigh less than 5 lbs.
U.S. Pat. No. 4,456,003 to Allard, et al. [21] used a spring across a knee joint to provide a knee-extension moment while permitting knee flexion and a mechanism to lock the knee in full extension. However, the device required a specified ankle angle in order to release the knee lock. Kofinan et al. used an elastic cord to resist knee flexion in stance and allow uninhibited knee movement beyond 25° of knee flexion [7]. The pre-stressed elastic cord was attached across the knee joint, anchored on the upper and lower sections of the SCKAFO. The eccentric knee joint was positioned posterior to the cable/elastic cord's line of tension. The eccentricity of the knee joint provided a greater knee extension moment due to the increased distance between the knee joint axis and the cable's line of tension. Knee flexion was constantly resisted by an extension moment created by the tension in the cable/elastic cord. When the knee was brought to 25° flexion, the distance between the cable's line of tension and the knee-joint axis equalled zero and thus the external moment imposed on the knee joint by the elastic was zero. Any further flexion of the brace beyond 25° caused the steel cable to wrap around a pin protruding from the knee axis. The pin held the cable's line of tension at the knee axis maintaining a zero external extension moment. This functionality also allowed for unassisted sitting while wearing the brace.
The theory followed that, in stance, loading of the braced limb would not cause the knee to flex beyond 25°; therefore, the elastic cord would provide a stabilizing extension moment throughout stance. One design aspect that differed from most SCKAFOs was that, as in able-bodied gait, some knee flexion in early stance would be permitted. In terminal stance, the knee would flex beyond 25° in preparation for swing. The external extension moment would disappear beyond 25° knee flexion allowing uninhibited movement of the knee in swing.
The design required a straight line of action for the cable and the spring. In order to achieve this, it was not possible to have the orthosis uprights follow the curvature of the limb and large spacers had to be used to anchor the orthosis upright to the AFO component. This made the device bulky medial-laterally. The brace was designed for children. Designing the brace to accommodate the higher knee moments generated by adults would have required even bulkier attachments for the spring.
U.S. Pat. No. 6,500,138 to Irby et al. discloses a SCKAFO, which integrated a conventional unidirectional clutch into the joint [3]. The orthosis is electro-mechanically actuated, using pressure sensors positioned beneath the heel and forefoot to detect heel strike and heel off. Integrated circuitry interprets signals from the pressure sensors and controls a solenoid which engages/disengages a wrap-spring clutch built onto the knee joint. The wrap-spring clutch uses a close-wound helical spring to transmit torque across a pair of mating concentric clutch hubs.
When the knee attempts to flex, the spring tightens over both concentric hubs, stopping relative motion between the two, thus preventing knee flexion. To disengage the clutch in swing, the spring is loosened. Loosening is achieved by pulling back on one end of the spring, called the control tang, via a solenoid. Though the electromechanical knee joint system proved to be quite effective, the device's excessive profile and bulk limits the practicality of the orthosis.
Myers of NASA developed a releasable conical roller clutch knee joint similar to an overrunning roller clutch design [8]. The upper and lower housing of the knee joint rotated about a needle bearing. The conical inner surface of the upper housing contained tapered pockets. Fixed to the inside of the housing was a cage retaining rollers in the tapered pockets. Springs influenced the rollers to remain in the wide ends of the tapered pockets. The conical inner surface of the lower housing was meant to contact the rollers when the upper and lower housings were brought together. A cam lay between the two housings of the knee joint, connected to an actuation rod, running to a heel-strike mechanism positioned near the foot. The mechanical heel-strike mechanism employed a series of levers and pushrods to convert foot pressure into a pulling force on the actuation rod.
At the beginning of stance, the heel strike mechanism converted foot pressure to a pulling force on the actuation rod. A pull on the actuation rod caused the cam to force the upper and lower housings together. The rollers were brought into contact with the walls of the tapered pockets and the conical inner surface of the lower housing. Flexion of the knee in this state would cause the rollers to roll into the tapered end of the pockets, and jam them between the conical inner surface of the lower housing and the walls of the tapered pockets. The wedging force would prevent the knee from rotating into further flexion. Knee extension in this state would cause the rollers to move to the wide end of the tapered pockets. In this state, the rollers were not subject to jamming and the knee was allowed to extend freely.
When foot pressure was removed at the onset of swing, a spring in the heel-strike mechanism forced the actuation rod upward. When the heel-strike mechanism pushed up on the actuation rod, the cam forced the upper and lower housings apart. No locking could occur in this state since the rollers were no longer in contact with the conical inner surface of the lower housing, and the potential of jamming was eliminated. The knee joint was therefore free to rotate in flexion and extension. Despite its apparent utility, the cost of machining the conical roller clutch device was too high for practical application. Furthermore, in order to withstand the stresses applied to the joint, the roller clutch would be excessively large and heavy.
Since January 2002, four SCKAFOs have been released on the market: Otto Bock's Free Walk, Becker Orthopedic's UTX and 9001 E-Knee, Horton Technology Inc.'s Stance Control Orthotic Knee and Fillauer's Swing Phase Lock. Though they have made it into the commercial market, these locking knee joints do not fully satisfy the size, weight, cost, function and cosmetic requirements of individuals that use these devices.
Manufactured by two different companies under two different names, the Otto Bock Free Walk and Becker UTX share the same ratchet/pawl design. A spring-loaded pawl locks the knee automatically when the knee moves into full extension prior to heel strike. To disengage the lock, 10% dorsiflexion of the ankle causes a control cable connected to the pawl to be pulled. Simultaneous knee hyperextension is required to nullify any flexion moment about the knee and, thus, free the pawl for disengagement.
The main disadvantage of this design is that full knee extension is required to engage the knee-flexion lock before weight bearing. The brace therefore provides no support to users if their knee is flexed when the leg is loaded, a common event in walking stairs, inclines, uneven ground or in stumbling and relaxed standing. This SCKAFO therefore does not practically serve many potential SCKAFO users that may be too weak to fully extend their leg while walking. The disengagement mechanism requires 10% dorsiflexion; therefore it cannot be used by patients with fused, deformed and spastic ankles. In addition, the delicate tubular steel structure may be unappealing to clients who feel they need more support [9].
Horton Technology Inc. produces Horton's Stance Control Orthosis as disclosed in U.S. Pat. No. 6,635,024. The locking mechanism is modeled after a standard unidirectional clutch design and involves an eccentric cam, which jams itself into a friction ring attached to the upper knee joint. The cam is connected to a pushrod, attached to a thermoplastic stirrup, which is displaced just below the heel of the client. Heel contact causes the stirrup to be pushed upward to engage the pushrod and nudge the cam into the upper joint head. The surface of both the steel cam and steel friction ring are textured with micro grooves. These grooves eliminate any slipping between the friction ring and the cam. When the cam is engaged, flexion will cause the friction ring to pull the cam into itself, thereby locking it. Knee extension will, however, cause the cam to be pushed away from the friction ring, and continue unimpeded.
Once heel contact ceases, a spring pushes the pushrod down, the cam disengages, and the knee is allowed to move freely. A hyperextension moment about the knee is required to eliminate any impinging force on the cam and allow it to disengage freely. The Horton Stance Control locking mechanism can also be outfitted on a KAFO with a free moving ankle. In this case, the pushrod is attached to the heel. Whenever the foot planterflexes (pointing toes downward), the cam is pushed upward to engage.
The orthosis as a whole is somewhat bulky [12] and the joints themselves are relatively large and heavy by KAFO standards. While Horton's Stance Control Orthosis does have the ability to lock at any knee angle, its weight and bulk are not well tolerated by many individuals using the device.
Both mechanical actuation methods used to control the device have their shortcomings. Objects such as clothing, or debris when walking outdoors, can become lodged between the foot and the stirrup. The bulky thermoplastic foot shell may prevent the client from donning a shoe, and the free-ankle option cannot be used by people with fused, deformed or spastic ankles.
In response to the limitations of the Stance Control Orthosis'mechanical actuation methods, Horton Technology Inc. had planned to release the Smart Knee—an electromechanical orthosis [13], which uses the same locking mechanism as Horton's Stance Control Orthosis, but replaces the stirrup and pushrod with pressure sensors below the foot and solenoids to actuate the lock. The Smart Knee was to be released in 2003 but has not become available as of 2006.
Basko Healthcare has developed a novel, gravity actuated, knee-joint locking mechanism for its Swing-Phase Lock orthosis as disclosed in published U.S. patent application Ser. No. 2003/0153854 [15]. For this device to function, a weighted pawl falls in and out of locking position, depending on the hip angle. When the hip is flexed anterior to the body, as in terminal swing, the weighted pawl falls into the locked position, preventing knee flexion. The knee must be fully extended for the pawl to fall into the locked position. When the hip is swung behind the body, prior to swing, the weighted pawl falls out of engagement and the knee is allowed to flex freely. A hyperextension moment of the knee is required to eliminate any impinging force on the pawl to allow it to fall out of engagement freely. The thigh angle required to engage and disengage the pawl is manually set on the joint head itself by an orthotist. Only one Swing Phase Lock is mounted on the KAFO. The other orthotic knee joint, mounted on the medial side of the KAFO, is a simple mechanism that uses friction and a spring to regulate knee flexion during swing phase [14]. As the locking mechanism is position dependent, this design is not effective for climbing stairs or walking on uneven ground. The joint only locks with full knee extension. This requirement limits where the patient can walk and provides no support if the patient stumbles in mid-step.
U.S. Pat. No. 6,517,503 to Naft et al. discloses Becker's 9001 E-Knee which is essentially a magnetically activated one-way dog clutch. The joint integrates two ratchet plates that are spring biased apart. One of the ratchet plates is positioned within an electromagnetic coil. When electric pressure sensors below the foot detect foot contact with the ground, the electromagnetic coil is energized and the ratchet plates are forced together. When engaged, the ratchet plates allow relative angular motion in only one direction. In stance, knee flexion is resisted while knee extension is still allowed.
Ratchet devices suffer from two inherent disadvantages including noise and a limited number of locking positions. Like a household ratchet tool, the 9001 E-Knee generates a clicking sound when rotated under engagement. The joint will therefore generate an unnatural ratchet sound whenever the user extends their knee in stance. Cosmetics are equally as important as function to KAFO users. If an orthosis looks or sounds unnatural, the device will draw unwanted attention to the user and the orthosis will not be used.
Unlike most friction-based clutches, a ratchet device only has a finite number of locked positions. The 9001 E-Knee houses 60 ratchet teeth, thereby allowing up to 6° of free-fall knee flexion before the joint settles into the locked position. Users that require the confidence of a rapid engaging knee lock will not tolerate this lack of support.
The 9001 E-Knee's biggest drawback is its size, weight and cost. Measuring over 2 cm thick, the 9001 E-Knee has a large profile that can be obtrusive and severely affect the orthosis' cosmetic appeal. The electromagnetic coil contributes to make the 9001 E-Knee the heaviest of all SCKAFO joints on the market. The joint's excessive weight places an unnecessary burden on the user, increasing energy expenditure during ambulation and leading to premature fatigue. The 9001 E-Knee is the most expensive of all SCKAFO joints, costing nearly double the price of other commercial SCKAFO joints.
The Free Walk/UTX [19] and Swing Phase Lock offer limited functionality as they both require the knee to be fully extended before they can provide support in stance. This is an unrealistic and potentially hazardous requirement as the user may load their leg with a flexed knee when climbing stairs, walking inclines or uneven surfaces, during relaxed standing or reacting to a stumble. Many KAFO users do not have the muscle strength required to fully extend their knee consistently during walking. A SCKAFO that requires full knee extension to activate any knee support jeopardizes the user's safety and mobility.
The key disadvantages of the Horton Stance Control Orthosis and 9001 E-Knee are their excessive weight and bulk. A major reason among clients for abandoning the use of long leg braces is that the assist device is too bulky and unpleasant for frequent use [16]. Potential users already suffer a physical weakness and will not wear a heavy SCKAFO that demands an excessive amount of energy to walk. Orthotic knee joints must also have a thin profile medial-laterally. An excessive profile can cause the lateral (outer) knee joint to collide with passing objects and the inner medial joint to rub against the opposite knee. If the user is wearing a brace on both legs, the inner knee joints could collide during walking. The excessive physical size and weight of the Stance Control Orthosis and the 9001 E-Knee make them too obtrusive and heavy for many users to tolerate.
Cosmetics are an extremely important issue for KAFO users. If an orthosis looks unnatural, sounds unnatural, or forces the user to move in an unnatural manner, the orthosis may not be used, regardless of how well it functions. The ideal orthosis should be unnoticeable under clothing and generate no noise. The excessive profiles of both the Stance Control Orthosis and 9001 E-Knee create a very bulky look even underneath clothing. The thermoplastic stirrup integrated into the foot piece of the Stance Control Orthosis adds further bulkiness to the braced leg. Because the 9001 E-Knee is a ratchet device, it generates an unnatural ratchet sound when rotated under engagement. For most clients, walking with a brace that generates clicking sounds is unacceptable. A practical SCKAFO, therefore, must be relatively silent.
There is therefore, a need for an articulating joint that delivers a combination of function and structure that permits natural gait and addresses the limitations of SCKAFOs currently known in the art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.